wireless: rndis_wlan: modparam_workaround_interval is never below 0.

priv->param_workaround_interval is unsigned, modparam_workaround_interval not.
the former is never < 0.

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>